1. Field of the Invention
The invention concerns a lamp, and more specifically pertains to a vehicle headlamp system which uses a discharge lamp as a light source.
2. Discussion of the Related Art
Conventionally, filament light bulbs have been used as light sources in vehicle headlamp devices. Depending on the light bulb structure, a four-lamp or two-lamp type headlamp device is used. For example, a four-lamp conventional type headlamp device can be constructed as shown in FIG. 8. The four-lamp type headlamp device 1 is equipped with light bulbs 2 and 3 which are installed side by side on both sides of the front part of the vehicle (only the right-side headlamp device is shown in FIG. 8). One of the light bulbs 2 is configured to emit a so-called high beam (hereafter referred to as "H beam"), while the other light bulb 3 is configured to emit a so-called low beam (hereafter referred to as "L beam").
The respective light bulbs 2 and 3 are accommodated inside a lamp body housing 4, and are supplied with electric power by a lighting device not shown in the figure. Furthermore, the light bulbs 2 and 3 are equipped with respective reflective members 2a and 3a so that the light emitted by the respective light bulbs 2 and 3 is reflected by the reflective members 2a and 3a, and is thus directed to the front of the vehicle.
In a headlamp device 1 constructed as described above, only the light bulb 3 is lit in the L beam mode, while both of the light bulbs 2 and 3 are lit in the H beam mode. As a result, the H beam light that is emitted frontward has sufficient light quantity, and a driver is comforted by the increased formward field of vision which is due to an increased quantity of light in the H beam mode.
A conventional two-lamp type headlamp device 5 can be constructed, as shown in FIG. 9. A two-lamp type headlamp device 5 can be installed on both sides of the front of the vehicle (a right side headlamp is shown in FIG. 9). Each of these headlamp devices 5 is equipped with a single light bulb 6. The light bulb 6 generally has a double-filament structure equipped with two filaments 6a and 6b for emitting the L beam and H beam, respectively. The light bulb 6 is accommodated inside a lamp body housing 4, and is equipped with a hood 6c used to block the light. The light bulb is supplied with electric power by a lighting device not shown in the figure. In a headlamp device 5 constructed as described above, power is switched to filament 6a in the light bulb 6 during L beam mode, so that an L beam is emitted from the filament 6a. During H beam operation, power is switched to the filament 6b in the light bulb 6, so that an H beam is emitted from the filament 6b.
In the case of a so-called H4 type halogen lamp, which is a lamp commonly used in the above-mentioned light bulb 6, the lamp is constructed so that a 55 W filament 6a is used for the L beam mode, and a 60 W filament 6b is used for the H beam mode, at the rated voltage. As a result, when the H beam is used, the illuminated area is larger than that illuminated by the L beam. Accordingly, any insufficiency in the quantity of light in the forward field of vision is compensated for by the 5 W increase in electric power.
In recent years, discharge lamps such as metal halide lamps, etc., which are advantageous in terms of brightness and lamp life, etc., have begun to see practical use as light sources in vehicle headlamp devices in place of conventional halogen light bulbs. In the above-mentioned four-lamp type headlamp device 1, a headlamp device 1 can be constructed by using discharge lamps instead of the light bulbs 2 and 3, respectively. It is also possible to change only one of the light bulbs 2 or 3 to a discharge lamp. The H beam/L beam switching can also be performed with high quality and accuracy, just as in the case when halogen lamps are used.
Discharge lamps are expensive compared to halogen light bulbs. Accordingly, it is preferable to use discharge lamps in a two-lamp type headlamp device 5 in order to reduce costs as compared to a four-lamp type headlamp device 1.
However, discharge lamps emit light by means of a so-called arc light source in which electricity arcs through gas and between two electrodes to cause light. It is physically impossible to install two arc light sources, e.g., for H beam and L beam use, inside a single discharge lamp arc tube. Accordingly, in a conventional two-lamp type headlamp device 5 using discharge lamps as shown in FIG. 10, the device is constructed so that H beam/L beam switching is accomplished by a method such as moving a portion 8a of a hood 8 to block direct light emitted from the discharge lamp 7, or moving the discharge lamp 7 itself.
Specifically, in the case of the L beam, a portion 8a of the hood 8 is extended as shown in FIG. 10(A), so that the light-blocking area is expanded. As a result, the direction of light onto region 9a of the reflective member 9 that reflects light into the distance is blocked, and light incident to oncoming and other vehicles is minimized. When the H beam is used, the portion 8a of the hood 8 is withdrawn as shown in FIG. 10(B), so that the light-blocking area is contracted. As a result, light is directed onto region 9a of the reflective member 9 to reflect light into the distance. Thus, the H beam light is emitted into the distance by the reflective member 9. The discharge lamp 7 is generally a 35 W lamp.
In a two-lamp type headlamp device 5 using discharge lamps 7 as described above, the L beam is very bright, and can sometimes cause the operator to feel that there is insufficient light quantity when the beam is switched to the H beam. In order to compensate for this uncomfortable feeling, a large auxiliary reflective part 9b is installed at the outside of region 9a that reflects light into the distance, as indicated, for example, by the dotted line in FIG. 10.
The above-mentioned two-lamp type headlamp device 5 is arranged so that switching between the H beam and L beam is accomplished with discharge lamps 7 equipped with a single arc light source. As a result, the operator may further feel that there is insufficient light quantity when the beam is switched to the H beam. The large auxiliary reflective part 9b on the reflective member 9 also eliminates this uncomfortable feeling of insufficient light which is due to the presence of a single light source that operates in both H beam and L beam modes. Accordingly, the overall size of the headlamp device 5 is increased, due to the inclusion of reflective part 9b, and the degree of freedom in vehicle design is restricted.